The present invention relates to the field of extrusion-blow molding machines for articles made of thermoplastic material, particularly for the production of containers such as bottles, flasks or water bottles.
Such a machine essentially comprises an extrusion head that produces at least one tubular plastic parison, at least one molding unit comprising a mold in two parts, the two parts of the mold being positioned so as to close over the parison that is still soft from the extrusion head, and a blow-forming station that is provided with means for Injecting air under pressure inside the parison enclosed in the mold. The air under pressure allows the parison to be deformed so that it fits the form of the mold cavity.
In some cases, the machine also comprises a degating station that allows the excess material to be cut off.
More particularly, the invention relates to a molding unit for such a machine. This molding unit comprises a mold in two parts In which each mold-half is borne by a movable support. The two movable supports are moved relative to each other along a path that is substantially transversal to the plane of the joint of the mold. In an open position, the two mold-halves are transversely separated from each other to allow a blank of the article, that is, a parison, to be inserted in a cavity delimited between the two-mold-halves. In a dosed position, the two mold-halves are pressed against each other by their front faces, and the supports are connected to each other by locking means.
The two supports can be movable in transverse displacement, but they can also be moved In rotation, for example, around a common axis parallel to the plane of the joint of the two mold-halves.
Of course, the molding unit has a mechanism for simultaneously moving both mold supports. This mechanism is adapted to the relative path of the two supports between their open and closed positions. In extrusion-blow molding machines, it is often provided that the molding unit comprises, between at least one of the mold-halves and the associated support, fluid pressure compensating means that push this mold-half transversely toward the other mold-half.
These compensating means make it possible to prevent the two mold-halves from separating from each other under the effect of the blow pressure. They also enable the front faces of the two mold-halves to be securely closed against each other In spite of the presence of the excess material caught between the two mold-halves at the use of closure. In some cases, the forces required to xe2x80x9csmashxe2x80x9d and pre-cut this excess material is quite large. They also depend on the quantity of material caught between the two molds.
An extrusion blow-molding machine incorporating these characteristics is described, for example, in the document U.S. Pat. No. 5,730,927.
In this document, it can be seen that the compensating means are in the form of hydraulic actuators, the machine being, in this instance, equipped with compensating means at both supports. One disadvantage of the actuator is that it only exerts a localized action on the mold-half. Such being the case, given the compensating forces applied, a localized action can only result In causing at least a local deformation of the mold-half. Also, in order to obtain a a certain distribution of the compensating force, six actuators are provided between each support and the associated mold-half, which make it possible to limit the deformations of the mold-half.
However, the actions of the actuators are still no less localized, and the mold-half must therefore have a relatively high rigidity in order for the force of the mold-half against the other mold-half to be close to homogeneous over the entire surface of the joint plane between the two mold-halves.
Moreover, this large number of actuators is a disadvantage in terms of set-up costs.
A more particular object of the Invention, therefore, is to propose a new design of compensation means that make it possible to obtain, with a simple, low-cost device, a very good distribution of the compensation forces over the entire surface of the mold.
To that end, the invention proposes a molding unit of the type described above, characterized in that the fluid pressure compensating means are accomplished in the form of an inflatable flexible cushion Interposed between a rear face of the mold-half in question and a front face of the associated support.
According to other characteristics of the invention:
in transverse projection the cushion extends in such a way as substantially to cover the entire surface of the transverse projection of the mold-half;
when the supports are in the closed position, the molding unit is ready for the blow-molding, and the inflatable cushion controls the movement of the mold-half from its retracted position to its advanced position after the two supports are locked in dosed position;
the mold-halves are furnished with precut means which, when the mold-half is in the advanced position, perform a pre-cut of the parison depending on the contour of the article to be formed;
the mold-half comprises controlled means for returning the mold-half to its retracted position;
the return means comprise an auxiliary inflatable flexible cushion; and
the return means comprise a bracket, which is placed at the outer side of the support and which bears, at its two opposite ends, connecting pins that extend through oblong apertures made in the lateral faces of the support in order to be connected to the mold-half, and the auxiliary cushion Is interposed between the bracket and a rear face of the support.
The invention also proposes an extrusion-blow molding machine, characterized in that it comprises at least one molding unit incorporating any of the preceding characteristics.